Standard signal generator



Sept 13, 1932 M. FERRIS STANDARD SIGNAL GENERATOR Filed July 13, 1929 3 Sheets-Sheet l M. FERRIS STANbARD SIGNAL GENERATOR- Sept 13, 1932.

5 Sheeis-Sheet 2 Filed July 13. 1929 mu (I zorriamuos Zlwuentok:

Sept. 13, 1932. FERRIS 1,877,237

STANDARD SIGNAL GENERATOR Filed July 15, 1929 s Sheets-Sheet s 7"o 209,000 Ml'croro/k;

Patented Sept; 13, 1932 UNITED STATES "RADIO CORPORATION OF AMERICA, OF NEW YORK,N. 'Y.', A CORPORATION 'IDELA WARE STANDARD SIGNAL GENERATOR Application filed July 13, 1929. Serial No. 378,091.

This .invention relates to apparatus for radiomeasurements, and more partlcularly. to apparatus .for the generation of radio frequency signals ofknown strength. 7

do f

:In the measurement of field strength and in thestestingof radio receivers, .a'controlled provide apparatus for thegeneration of signals of :known .radio frequency microvoltages. An objectistoprovide apparatus for use inthe testingof radio receivers todetermine, quantitatively, their performance, and particularly their sensitivity, selectivity and fidelity I of reproduction.

.A further object is to provide a sourceof radio'frequency signals of predetermined and controlled radio frequency voltages andhavinga predetermined and controlled percentage of modulation. A further object is to provide a signal generator of the type stated in which all parts.except,.if.desired,the energizing batteries, maybe located n sh elded housing or box, the construction being such that the only radio frequency output from the shielded housing is the measured and accurately predetermined output which is avai ablea t e output signal terminals of the generator. Further objects are to improve the. shield ng f and betweenthe component parts of the oscillation generator, and particularly the shielding of the attenuator which is employed to vary the magnitude of the radio frequency output.

These and otherobjects of the invention will be apparent from the following description .when taken vith the accompanying drawings, in which i Fig. lis a diagrammatic representation of .the electrical-circuits and the shielding employed in .one form of signal generator embodying the invention, Fig. 2 is a front elevation iofthe instru ment panel for the same, l

3 is a circuit diagramof a voltage'fi attenuator system, and

Fig.4 is a side elevation, Withpartsshown I in section, of the shielded attenuator switch.

Before proceeding with thedescription of the apparatus, attention is .dirested to the fact that there is no problem involved in the design and construction of an oscillatorfor producing modulated or .unmodulated radio frequency signals, since thedesign, construc tion and operation of such oscillators are well known in the art. The real problem relates to the production, of signals of predetermined and adjustable magnitude. In

the testing of radio receivers,'standard signals of as low as one-microvolt are desired,

but it is impossible to make a direct and emu F E I MALCOLM PEER/IS, OFBOONTO'N, NEW JERSEY,'ASSIGNOR, BY MESNE ASSIGNMENTS TO i accurate measurement of voltages ofthis;

order of magnitude.

; Inaccordance w th the present nvention,

a signal generator, which is, or may beef:

any conventional design, is associated with electrical indicating instruments of the ori dinary type and with graduated adjustable devices, the entire assemblybeing soconstructed and arra ged that. the signa n rgy.

available at the output terminals of the gen-. era-tor assembly may beset atany'desired.

and accurately predetermined value by an appropriate adjustment (as indicated by the graduations) of the variable devices, and that substantiallyno signalenergy will escape from the generator as,stray energy.

As will be explained nereinafter, the solution of the problem is based primarily upon the employment'of shielding and filtering of a highly developed character. Y a

In Fig.1 of the drawings, the main shield or casing which houses the entire assembly, is indicated by the reference numeral 1,.the

radio frequency oscillator tube2, the audio frequency oscillator or modulating tube 3,. and the associated circuits are housed within a shield 4 which comprises a complete housing located within the outer housing 1. As indicated above, the particular sir.- cuits employed in connection with these oscillators are capable of wide variation and in the embodiment which is shown by way of illustration, the radio frequency oscillator tance L and capacity C may be simultaneincludes a plate circuit in which the inducously Varied by a common operating shaft This arrangement is not necessary, but is preferable since it allows a design which will give a substantially constant output current at various wavelengths and thus requires only connected across the appropriate terminals;

8, 8, on the panel. The plate circuit of the radio frequency oscillator includes the secondary 9 of a transformer having a primary 10' for association-with the external oscillator anda primary 11' which is connected across the voltage divider 12 in the output circuit of theaudio oscillator 3. I

A thermo-couple 13 isconnecte'd across the transformer winding 11 and located within theinner housing-a. The leads from the thermo-coupl-e to the voltmeter 1 1 are located in the space between'the housings 1 and 4, and

include series chokes l5'andshunt capacities 16. Filament current for both tubesls sup-- pliedfrom an external battery A which may be connected across the terminals 17 of the panel, the negative lead being grounded through the control switch 18 and the positive lead including a filter 19 which is located between the twocasings. Plate current is supplied from an external battery B connected across terminals 20 and the positive lead in-- clud-es'the-filter 21 in the space between the casin'gs. t v 1 The output from-the oscillator tube 2 is through lead 22 and thermo-couple 23 to the voltage attenuator, an ad ustable resistance 24 bemg provided for regulating the current passed through the thermo-couple and attenuator.- resistance 24, as well as a part of-thefilter 25, in the leads to the ammeter 26 are shielded by a housing 27 which is completely enclosed by a main housing 1. The second section 28 of the filter in the ammeter leads lies between the shield 27 andthe main housing 1. 1

- The voltage attenuator comprises a network of resistances and a pair of switches includingthe switch arms 30 and 31 which are connected generator.

paratus are connected to slide wire 33 and contact 34 to serve -as output terminals for the The relative Values of the resistances and their connections to the switch points are such that movement of switch arms 30, 31 from one position to an adjacent one will result in a he lead 22, thermo-couple 23 and decimal change of the voltage across the slide wire. As shown in Fig. 1, the switch is so positioned that the minimum Voltage is avail able at the output, and this voltage will be multiplied by ten for each step that the switch tenuator switchcsfwhenthe attenuator is adequately shielded. The perfection of shielding which'is'essential for satisfactory opera tion is, however, of an order decidedly higher than anything previously employed.

The slide wire 33 is positioned adjacent-one wall of the outer casing 1, being preferably located just back of the front wall or panel 1 of the casing, and is shieldedby the-subpanel. 36 which is .of substantially greater extent than the slide wire. shell 37 cooperates with the 'subpanel 36 to provide a shield for;

the two preceding elements ofthe resistance network, the three preceding resistance elements being shielded by the shell 38 which houses the shell 37. The leads to the two ter-' minal sections of the attenuator are shielded by tubes or braid 39, see Fig. 1, andarcuate plates 40 are provided at both sides ofthe' switch points to avoidany substantial capacitivecoupling between the switch arm 31 and the contacts. The two switches are separated by a shield 41 which cooperates with plates 42, inner casing l and shells 37,38, to complete the housing or shield for the attenuator. It is to be noted that the shield 37 for the term had or lower voltage section of the attenuator network is located within the shell 38 which houses'a higher voltage se'ctiomand that the lead to the slide wire is brought out of the attenuator shield at a point on subpanel which is within the double shield.

The front panel 1 may beof metal, for example aluminum, and form a part of themain housing. The particular panel arrangement which is shown in'Fig. 2 has been employed with a generator havingthe circuit arrange ment shown in Fig. 1. The instruments and terminals are identified; by thereference numeralsof the Fig. 1 diagram, and the operating knobs and dials are identified by prime numerals corresponding to the numerals off the respective adjustable elements .of the circuit.

As shown in Fig. 1, a radiov receiver R is connected across the output terminals 35 of the generator through the composite impedance Z which simulates an antenna impedance. g

The operation of the generator is charaoter ized by its extreme simplicity. The batteries, the-receiver R and the audio oscillator7, when one is used, maybe located inany convenient position; Upon closing switch 18, the radio oscillator 2 is energized and the "wavelength ofit'he' generated signals may bead justed dial 5; and pointer 6" For continuous wave or foreXternal modulatiom swltch 3 remains The modulating voltage; as 1 indicated by the instrument 14, maybe ad usted to the desired vaiueby knob l2 'inthe case-of; internal mod.

ulation and by the usual control devices forming part ofthe oscillator =7 when external modulation is employed.

Thesignal energy delivered to the attenuatorisadjusted, by knob, 24 and rheostat 24, to a definite value, say milliamperes, which is indicated by the milliammeter 26. The voltage available at the output terminals is determined by the adjustment of the switch knob 32 and slide wire knob 34, and its measured value, in microvolts, is equal to the prod uct of values indicated on the scales associated with these knobs. V

For the particular values indicated upon the panel, the resistances of the elements of the attenuator network will have the respective values indicated on the schematic diagram of Fig. 3, and the measured output may be varied between 1 and 200,000 microvolts.

The necessity for substantially perfect shielding will be apparent from a consideration of Fig. 3. A mathematical analysis of the resistance network shows that when the switch arms are set to establish the minimum voltage across the slide wire, the current es tablished in the slide wire by the conductive .'-:network is only 10 microamperes and that an error of one percent would be introduced if the phantom capacity G between the slide wire tap and switch'arm 31 were as great as mmf. The resistance elements of the network are, of course, non-inductiveand may be wound of #38 Manganin wire, or smaller.

While the invention is not limited to the employment of circuit elements'or of currents of the values stated above, these values have been found to be satisfactory for a generator intended for the testing of radio broadcast receivers. Normal output may be had with most receivers when the detector input is 200,000 microvolts. With an input of 100 milliamperes into the attenuator, a relatively rugged type of switchboard instrument is satisfactory. A much smaller current would make necessary the use of a more delicate measuring instrument, and would limit the 'maximum output across the slide wire. An

appreciably greater current would necessi-l tate a more powerful oscillator andwould render the design and mechanical construction of the shielding more diflicult.

While the mechanical arrangement of the shields may be varied to a considerable extent, I believe that to secure a compact construction in a generator delivering accurately *measured voltages of the order of microvolts,

portancez (a) An quency oscillator; I i i inner "shield; for .1 the radio fre-M i All battery and instrumentleads 70 be carefully filtered,.thefilter "elements being locatedlbetween the inner and outer shields 0) .attenuatori shield between theat V are located 'bet'w'een the inner and outer I tenua'tor anduothencircuit elementswhich? '(d) Anninner shield :separating 'therlower voltage". sections of Y the: attenuator-u for the: other elements located. within the mainattenuator shield, the" output lead passing. from so H the main attenuator shield at a point-which is;

within both. attenuator shields; a

(e) Shielding betweenthe'gangedswitches,

when a .plurality of switches areemployed' 'inthe attenuator, and shielding ,be I

- tween the terminal 'switcharms and its con tacts; f inner and outerv shields of the-oscillator, and broken by insulatingcouplings By giving due consideration tors, the voltage across the slide iwi-re has the value' indicated by; the instrument settings,- I and all 'computationsand indirect measuring: methodsare avoided. FT-he perfection of file teringand shielding, is,; of course, dependent Control-shafts grounded to both the i upon the degree of accuracyswhich is re-I quired. For relatively low maximum outputs and/or where highaecuracy-is not necessary, all of the precautions; noted above are" not essential.

It is therefore bod-iment which I: have described, since va -b un ersto dthat the j VGlltiOn. not r cted to Specific. em-- I my inventionas set forth in: the renewin f claims.

I- cla m :2

l. y The combinationw-ith an: oscillator for l ne generating radio frequency signals, a voltage attenuator comprisinga resistance network and anadjusting switchltherefor, and means. including anlindicating instrumentifor ad justing the'signal energy deliveredrto said attenuator to a predetermined value; of metallic spacedcasings providing a doubleshield about; said oscillator, a metallic casing positioned in the int'erspa'ce of said'doubl'e shield casings and providinga shield for said attenuator, andshields within sai dflastmentioned "casing for "substantially 'eliminating coupling between theterminal resistance" thereof and the other parts of said attenua tor,-wh'ereby the voltageavaila'ble acrosssaid terminal resistanc'e "isi predetermined bythe adjustmentiof said attenuator switch;

2; The inventionj as set forth inzclaim hiin combination-withasoiurce of. 'currentsupply exterior to said double shield for energizi'ng-z sa-id oscillator, andsa, filter, networlzlocated V in the interspace of said double shield :for preventingsigrial energy from passing outside said double shieldby'way of said ourrent'supply-leadsr.

from said generator, of a voltage attenuator comprising a'plurality of resistanceelements and a pairof multipointswitches, of a shield ments of saidnetworkfand; the-leads connecting said terminal "elements with their respective switchcontacts, and a shield cooperating with saidswitch contacts to substantlally eliminate electrostatic coupling between 1 said switch contacts and their" associated switch armjr said shields;-

bination of an oscillator for generating; radio frequency, signals, and an inner and outer sh eldhouslng said'oscillator, sa1d inner and I outer shields being in spaced relationship, of

"said "outer shield, leadsextending between said instruments and said "oscillator,'filter' elements in said'leads and located between,

said shields. 7. The invention as set forth in claim 6, in "combination with terminals at the exterior of said'outer shield for connection to sources housing at a point within bothsaid "radio frequency signals, and metallic shields 1 preventing the escape of'straysignal energy preventing coupling between" said switches, shields enclosing the terminal" reslstance ele- 6: Ina standard signal generaton'the commeasuringinstrumentsicarried-by a panel of signal. generator comprising aslide Wire and 9. A voltage attenuator, fora standard r a pluralityof resistance elements, a shield for said slide wire, a metallic casing enclose V ing said elements, a shield within said casing and separating the lower voltage resistance elements from the preceding elements, and a lead from said low voltage elements to said 1 slide wire, said lead extendin shield and said housing. i o

In testimony whereof, I affix my signature.

MALCOLM vFERlEtIS. v

of current supply, leads extending from said terminals to said oscillator and including filter elements 8. Ina standard signal generator, the comlocated in the'space' between bination with an oscillator and spaced inner V and outer shields housing the same, of a space :between said shields, a metallic housing enclosing said attenuator, and a metallic shield within said housing and" separating the low voltage resistance elements from the flflzhigher voltage resistance elements, a switch for establishing connection between said oscillator andipointsc in said network, and shields housing theleadsifro mzsaid switch t I oithejlo w er voltage elementsof said netiw --work, 13, r i

yoltage attenuator network positioned in the. v

from said 7 QER'HFIGATE OF CORRECTION.

Patent No. 1,877,287. September 13, 1932.

MALCOLM FERRIS.

It. is hereby certified that error appears in the printed specification of the above numbered paient roqnii'ing correction as follows: Page 2, line 101, for the words "term ham?" read "terminal"; and that the said Letters Patent should be read with this oorromioz'i therein that the samomay conform to the record of the case in tho Fatsnififlioe. v

Signed and sealed this 28% day of February, A. I). 1933.

M. J. Moore,

' Seai) Acting Commissioner of Patents.- 

